Electric discharge device



Jglyll, 1933. s. P. SASHOFF v $917,997

amcwruc mscrmner: DEVICE Filed May :51. 19:50 s Sheets-Sheet 2 59 R018. volts.

fp-RMS. Volts.

R2 in Mega/7212s.

INVENTOR Stephan PSqs/vo/fi BY zATToRNEY Julyll, 1933. s. P. SASHOFF ELECTRIC DISCHARGE DEVICE Filed May 31, 1930 3 Sheets-Sheet 3 ATTO'RNEY INVENTOR Stephan P505720 II II 3 l i v $111,114.49

G2 in mfas.

Temp 0 Figjfi" Distance Pressure.

Fig 12.

l l B 0 0 o 0 0 ..H 0 QR KQ W 9 0 0 0 0 2 0. 0 RQQQ m K \N Patented July 11, 1933 UNITED STATES PATENT OFFICE STEPHAN P. SASHOFF, OF SWISSVALE, PENNSYLVANIA, ASSIGNOR 'IO WISTINGHOUSE ELECTRIC 8a MANUFACTURING COMPANY, A CORPORATION OF PENNSYLVANIA ELECTRIC DISCHARGE DEVICE Application filed Kay 81,

My invention relates to an electrical discharge device and especially toa hot-cathode gaseous discharge tube.

An object of the invention is to provide an electrical discharge tube capable of handling comparatively large currents when operated as a relay or rectifier.

An additional object of the invent-ion is to provide a discharge tube that shall .be very speedy, extremely sensitive, and without moving mechanical parts while acting as a relay.

More specifically it is an object of the invention to provide a grid-controlled hotcathode discharge tube capable of utilizing commercial voltages and currents in its operation as a relay or rectifier.

Mechanical relays of the prior art have the objection of being slow in action and of lacking sensitivity. Furthermore, their moving mechanical parts presented a problem of replacement with wear in addition to the problem of maintaining these moving mechanical parts in the required adjustment. Thermionic devices have been utilized, but due to the fact that these thermionic devices are not generally capable of operating at the usual commercial currents and voltages, it was necessary to use them in combination with a mechanical relay that could be operated with commercial currents and voltages. Accordingly, I have developed an electrical discharge device, that is capable of direct use as a relay in place of being an auxiliary to a mechanical relay. Furthermore. such device is extremely sensitive and practically instantane'ous in action.

The particular form of my invention is an electrical discharge device of the type commonly referred to as a gaseous discharge tube. I provide in this gaseous discharge tube a control grid preferably spaced from the anode a distance of the same order as the mean free path of an electron in the gas in the tube. Furthermore, I provide a hotcathode, preferably of the filament type, for providing electrons for ionizing the gas within the tube. This tube is capable of handling a current of the order of anywhere from an ampere to 6 amperes or more instead of the 1980. Serial No. 458,850.

tion taken in con unction with the acc0m panying drawings, in which,

Figure 1 is a view, partly in cross-section and partly in elevation of a preferred form of my power-grid glow-tube.

Figs. 2, 4, 6, 8, 10 and 12 are graphs disclosing the characteristics of the tube in various circuits.

Figs, 3, 5, 7, 9, 11 and 13 are circuit diagrams, illustrating applications of the tube.

Fig. 14 is a sectional view, of an anode structure modified from that disclosed in Fig. 1.

Fig. 15 is a curve disclosing the breakdown voltage in respect to the product of electrode spacing and gas pressure. This curve determines the spacing of the electrodes as hereinafter explained.

The tube disclosed in Fig. 1 comprises the usual bulb or container 20, supported on a.

base 21 from which project the usual prongs 22 for the exterior connections of the tube. A press or seal 23 supports the connections from the exterior prongs 22 to the interior elements of the tube. In the center of this glass press 23 is preferably an upwardly projecting glass sleeve 24, surrounding the connection 25 to the anode 26. This anode 26 may be of any suitable material, such as nickel, iron or molybdenum. This anode may be screw-threaded to the top of the connection 25 or may be otherwise secured thereto. The glass sleeve 24 has preferably an exterior cylindrical surface.

A spacing insulator 27 is preferably slipped over the anode 26 into frictional engagement with the glass sleeve 24. This insulator may be of porcelain, soap-stone or ceramic material.

The shape of this insulator is substantially that of a hollow cylinder. The upper end 28 of this cylinder rests on the upper surface 29 of the anode, and has a small perforation 30 through this upper end 28. A control grid 31 is placed on the exterior surface of the end 28 of the insulator, and is preferably in the form of a cap fitting over this upper end 28. This cap is preferably of nickel, and, also, has a perforation 32 in alignment with the perforation on the end 28 of the insulator 27. A connection 33 is fastened by leads 34 to the grid-cap and extends through the glass seal to one of the exterior prongs 22.

Glass sleeves 35 and 36 also project upwardly from the press 23 to surround connections 37 and 38, projecting to the upper part of the tube from two of the exterior prongs 22. A cathode 39 is connected across these two connections and is adapted to be heated to an electron emissive state by the current passing through the cathode from these connections 37 and 38. This hot cathode 39 is of the filament type, and is preferably a metallic ribbon of Konel (a cobaltnickel-ferrotitanium alloy described in the co-pending application of Edwin F. Lowry, S. N. 403,664) nickel or platinum coated with an electron emitting substance such as barium-oxide. The shape of this hotcathode is preferably that of a horizontal coil with its axis spaced from, and preferably parallel with, the adjacent surface 29 of the anode 26.

The glass sleeves in the tube are to provide long high-resistance paths in case there is any leakage from one electrode to another across the surface of the glass.

The interior of the tube 20, after being evacuated, is filled with gas of suitable pressure. This gas is preferably neon, although argon or helium could be used. In place of any of these gases a small amount of mercury may be placed in the tube and part of it is evaporated so as to fill the tube with mercury vapor.

The thickness of the upper end 28 of the insulating tube 27 is designed to accurately space the grid cap 31 from the adjacent surface 29 of the anode so that the desired control will be effected. The distances between the electrodes are very carefully chosen. In Fig. 15 is disclosed a curve wit the product of distance between electrodes and the gas pressure as abscissa and the breakdown voltage therefor as ordinates. Either the distance or the gas pressure can be made a fixed value and the most desired magnitude of the other may be determined from the curve. In this case, the pressure of the as inside the glass tube is predetermined. enerally this pressure is between one half and 15 millimeters of mercury and preferably about 1 to 4 millimeters.

As the pressure of the gas is a constant, the curve in Fig. 15 discloses the breakdown voltage for various spacings of the electrodes in the tube. It will be noted that as the distance between the electrodes is decreased,

the breakdown voltage decreases until a minimum point A onthe curve is reached. As the spacing of the electrodes passes through the mean free path distance of an electron of the gas, the magnitude of the breakdown voltage increases very rapidly.

It is desired to have the tube breakdown between the grid and cathode substantially at the lowest possible voltage. Accordingly, the point A is projected on the two axes and the abscissa OA gives the distance for spacing the grid and anode for breakdown at the minimum voltage at the magnitude of voltage indicated by the ordinate OA.

It is also desired to prevent breakdown between the grid and the anode. Accordingly, a safety voltage value or maximum operating voltage is selected which is as high or higher than any voltage apt to be applied across the grid and anode of the tube. This value may be, for instance, five times the recommended grid-anode voltage. This value is determined on the vertical axis by the point B, for example, and then the point B is projected at right angles to the axis until it intersects the curve at the point B. The point B is then projected at right angles to the other axis until the projection intersects it at point B. The distance OB then represents the maximum spacing distance between the grid and anode for the maximum operating voltage. The distance between the grid and anode is, in general, less than one fourth the distance between the grid and cathode.

The thickness of the end 28 of the insulator very accurately spaces apart the anode and grid by the small distance determined from OB" of the curve in Fig. 15.

When the cathode 39 is heated to a given temperature either by direct current from a storage battery, for example, or with alternating current from the secondary winding of a filament transformer, it will begin to give off electrons. If there is a potential difference between the cathode 39 and the anode 26 these electrons will move towards the anode. When these electrons collide with the molecules of gas in the tube 20, they i onize this gas and produce new electrons and positive ions. The electrons are most abundant at the anode and positive ions are most abundant at the cathode.

At a certain potential of the grid, the electrons and positive ions arrive at the grid at equal rates and accordingly the grid will not receive a current. If the grid is made more negative than this critical value of potential, some of the electrons are repelled, and more positive ions attracted with the result that there is an excess of ions adjacent to the grid. If the grid is made more positive, positive 'ions are repelled and electrons attracted with the result that there is an excess of electrons near the grid. These regions of excess positive ions or electrons are called positive and n ative space charge layers, respectively, aiid constitute the onl isturbance that a grid can possibly ma e in the dischar e itself. If the discharge is of any appreclab e intensity these space charge layers will be very thin and cause little, if any, change in the glow or arc characteristic. At small intensities these space charge layers become thicker because t e condition that enough current be collected by the grid to establish its staple potential, requires that current be drawn from a greater volume of the discharge. At extremely small currents, such as precede breakdown, weak layers become very thick and constitute a very efiective barrier to the initiation of the discharge. It is characteristic of these devices that a discharge can be prevented but not stopped, except by interrupting the anode potential.

On alternating current or a pulsating direct current potential the discharge goes out on each zero-point of the wave and the grid can be used to either prevent or allow the discharge to start on the next cycle. On direct current the tube may be held in an open circuit condition indefinitely by the proper grid-bias until the voltage surge or some other disturbance momentarily changes the grid voltage and causes current to pass.

The tube may be made to operate either by the magnitude of the grid input in the case of direct current or by magnitude or the phase relation in the case of alternating current.

Various characteristics of the tube are disclosed in the graphs of Figs. 2, 4, 6, 8, 10 and 12. These graphs are taken from the operation of the tube in the circuits of Figs. 3, 5, 7, 9, 11 and 13, respectively. The potential of the grid can be referred either to the anode or the cathode. It has been common practice with the grid-glow tube to refer this potential to the anode and the characteristic curves are plotted for this condition.

In Fig. 3 a direct current voltage is applied between the anode 40 and the cathode 41. The potential on the grid 42 is obtained from the variable point on a 5000 ohm resistance potentiometer 43. The resistance R is used to limit the peak of the plate current, R is the grid leak resistance inserted between the grid and the variable point on the potentiometer. In Fig. 2 are curves showing the relation between the grid potential and the plate potential for various .values of the grid leak resistance. This potentiometer method of grid control has been found very satisfactory in many applications, where it had been desirable to select only a given portion of a pulsating signal.

In Fig. 5 the direct current voltage on the anode is replaced by an alternating current voltage 44 while the direct current bias 45 is supplied to the grid. A capacity 46 is shunted between the grid and the cathode. The curves of Fig. 4 are obtained for different values of a condenser used to provide this capacity between a grid and a cathode. The eflect of this condenser 46 is to change the phase angle between the anode-cathode and anode-grid voltages and thus chan e their magnitude for which breakdown will occur. These and other circuit conditions that are to follow are of great importancefor they constitute very convenient methods of phase shifting by means of which the power output of the tube can be controlled. R in this circuit has been made 3 megohms while R has been used to limit the current peak to an ampere, as a ampere gridglow tube is used in this circuit.

In Fig. 7 the direct current bias of Fig.

5 is replaced by an alternating current volt-' age obtained from the secondary 47 of a transformer, the primary of which is supplied from the same source supplying the anode-cathode potential. The curves of Fig. 6 are obtained for various values of the condenser shunted between the grid and the cathode. The curves are for zero-phase difference between anode-cathode and grid-bias voltage. They are seen to be straight lines e rcept for large values of positive bias. This c1rcu1t or a modification of it can be used to perform various operations at the instant of appearance of a predetermined value of grid-controlled potential. A typical example is the starting of a cathode-ray oscillograph within the fraction of a micro-second after a lightning disturbance on a transmission line.

In Fig. 9 the tube is made self-biasing through a resistor 48. The curve for various values of the condenser 49 between the grid and cathode are disclosed in Fig. 8. The anode voltage is plotted against the biasing resistance in megohms.

In Flg. 11 the potential of the grid is estabhshed by means of a potentiometer arrangement of condensers 50 and 51. A current limiting resistor R2 is used in the anode circuit and a 3 megohm resistance is inserted between the grid and the connection between the two condensers. In this circuit the rootmean-square value of the breakdown voltage is controlled by the size of the condensers used. In Fig. 10 the various curves for values of capacity between the grid and the cathode are plotted in respect to the relation of the capaclty between the anode and grid in microfarads, to the root-mean-square voltage of theanode. It is seen that the sensitivity of the tube is the greatest for very small values of two capacities, and a very small change in the value of the condenser 51, between the grid and the cathode is suificient to cause the tube to discharge. This might be done by waving ones hand over the tu The curves disclosed show the conditions f or which breakdown of the tube will occur for a few of the circuits. Other circuits could be employed such as by using a photo-electric 5 tube in place of the resistor 48 in Fig. 9. No

matter what circuit is used, a definite grid condition must exist if the tube is to breakdown for a given value of the anode-cathode voltage. When this condition is established the tube will pass current in the form of an arc. After the arc has started, however, the grid is surrounded by a space-charge which prevents it from exercising any further control over the discharge. v

The temperature characteristics of a tube filled with neon gas and a tube filled with mercury vapor are disclosed in Fig.,12. The circuit from which these values were obtained is disclosed in Fig. 13 in which therewas an 85 volt bias on the grid, a resistance R of 3.36 megohms, and a capacity of .005 mfd. shunted between the grid and the cathode. It will be noted that the temperature has no effect on the neon tubes, but has a decided effect upon the mercury vapor tubes. Accordingly, this curve for the mercury vapor tube indicates an application as a temperature relay for the mercury vapor tube.

Various modifications may, of course be made in the structure of the various elements of Fig. 1. Fig. 14 discloses a modification of anode structure. A hollow insulator 61 is used to support and space the anode and grid structures. The anode connection 62 extends upward to the central portion of this insulator, and a plug or cap 63, either iron or nickel, preferably with the screw stem depending therefrom, is attached to the top of the connection 62. A nickel cylinder 64 is secured to this cap and hangs downward with its lower end fixed in position by a flange 65 on the insulator. A grid 66 in the form of a screen or perforated cylinder closed at one end is supported on another flange 67 of the insulator. This screen is spaced from the anode by a distance OB determined from Fig. 15. If desired, a carbon block could replace the nickel cylinder. This structure, in

general, is able to carry more currents than that disclosed by the tube in Fig. 1.

I do not wish to be restricted to the specific structural details, arrangement of parts or circuit connections herein set forth, as various other modifications thereof may be effected without departing from the spirit and scope of my invention. I desire, therefore, that only such limitations shall \be imposed as are indicated in the appended claims.

I claim as my invention:

1. An electrical discharge device compristhereto, a perforated insulator on the surface of said anode adjacent said cathode, and a control electrode on said perforated insulator.

3. An electrical discharge device compris- 1ng an anode, an incandescible cathode spaced from said anode and substantially normal thereto, a perforated insulator on the surface of said anode adjacent said cathode, and

a control electrode on said perforated insu-- lator, in the form of a perforated cap.

4. An electrical discharge device comprising an anode, an incandescible cathode spaced from said anode and substantially normal thereto, a perforated insulator on the surface of said anode adjacent said cathode, and a control electrode on said perforated insulator in the form of a perforated cap the perforations in said insulator and cap being aligned.

5. An electrical discharge device comprislng a container, a gas enclosed by said contamer, an anode, an incandescible cathode and a grid, said grid and said incandescible cathode being spaced from one another at a distance for breakdown of said gas at substantially minimum voltage across said grid and cathode.

6. An electrical discharge device comprismg a container, a gas enclosed by said contamer, an anode, an incandescible cathode and a grid, said grid and said incandescible cathode being spaced from one another at a distance for breakdown of said gas at substantially minimum voltage across said grid and cathode, and said anode spaced from said grid at a distance less than that for breakdown of said gas at the maximum operating voltage across said grid and anode.

7. An electrical discharge device comprising a container, a gas enclosed by said contalner, an anode, an incandescible cathode and a grid in said container, said grid being spaced from said anode by a distance less than one fourth the distance between said scribed my name this 29th day of May 1930.

STEPHAN P. SASHOFF. 

